1. Field of the Invention
The invention pertains to means for the compensation of undesirable errors or changes in the signal characteristics of flux valve data repeater systems and more particularly relates to apparatus for the correction of variations in the outputs of flux valve compass data repeater systems, including errors due to variation in the horizontal component of the earth's magnetic field, index angle errors, and cardinal and intercardinal heading errors.
2. Description of the Prior Art
When navigating at high latitudes with flux valve magnetic compass systems, difficulty is experienced because of the decreasing strength of the horizontal component of the earth's magnetic field, especially at high latitudes. A flux valve magnetic compass is normally arranged to sense only the horizontal component of the earth's field. As a consequence at high latitudes, the strength of the sensed component is proportionally lessened, and the compass system experiences decreasing sensitivity, resulting in heading information of diminished accuracy.
Prior art systems have sought to solve this compensation problem of providing an input of the magnetic compass data repeater substantially independent of variations in the strength of the horizontal component of the earth's field, as by controlling the gains of amplifiers or the effective values of impedances in the separate channels of the data transmitter system in a relatively complex manner, but generally in inverse relation to the signal strength as measured at the flux valve itself. Such prior art arrangements are described by D. A. Espen in the U.S. Pat. No. 3,548,284 for "Synchro Data Transmission Apparatus Having Discrete Gain Changing to Compensate for Undesirable Signal Gradient Variations", issued Dec. 15, 1970, and by J. R. Erspamer and G. W. Snyder in the U.S. Pat. No. 3,646,537 for an "Automatic Gain Control for an Electromechanical Transducer", issued Feb. 29, 1972, both patents being assigned to the Sperry Rand Corporation. While these concepts have been useful in providing adequate magnetic field compensation in most circumstances, the compensating signals actually compensate only for variation in the horizontal magnetic field component and generally do not additionally correct fully for gain changes caused by component variations or due to temperature or power supply voltage drifts or to component aging. Furthermore, the characteristics of the individual gain control elements of the individual channels of the data system may vary without proper corrective relative adjustments whereby two-cycle transmission errors are induced within automatic gain control stages.
The improved system disclosed by J. R. Erspamer and G. W. Snyder in the U.S. Pat. No. 3,784,753, issued Jan. 8, 1974 for a "Multiplexed Gain Control for a Synchro Data Transmission System" sought more fully to overcome these prior art defects by a relatively complex and expensive correction circuit. Though it generally overcame such defects, it was found that some undesirable two cycle error could be generated in its relatively complex automatic gain control stage, and that a simple way was needed for identically changing the gains of both of the sine and cosine channels of the data transmission system, but retaining the advantages of the concept of U.S. Pat. No. 3,784,753.
Prior art systems have additionally sought to provide correction for the cardinal heading error in compass data transmission systems by use of networks including precision differential synchros or ganged dual potentiometers which must track each other with high precision if they are not themselves to introduce errors. According to the present invention, the expense of obtaining such selected synchros or precision potentiometers is desirably eliminated. Index angle error was similarly corrected in prior compass data transmission systems by using precision synchros or ganged dual potentiometers of similar quality. It is found increasingly desirable to substitute simple and less expensive networks permitting single adjustment control for each of these correction purposes and, at the same time, retaining a high degree of precision.